Various kinds of radio receivers include an RF amplifier, the gain of which can be automatically reduced under strong signal conditions by the use of AGC. Without such AGC, circuitry receiving the output of the RF amplifier can become overloaded, resulting in poor inter-modulation performance and other undesirable results.
A conventional RF amplifier 10 with AGC is shown in FIG. 1. Transistors 12 and 14 are interconnected as a cascode amplifier, with the transistor 12 receiving an RF input signal to be amplified, and an amplified output signal being developed across a tuned circuit 15. An AGC signal is coupled via an RC network 16 to the base of transistor 14. As the AGC signal goes low, the collector-emitter voltage of transistor 12 is reduced. This causes the transistor 12 to begin to saturate, thereby reducing the amplifier's gain and the amplitude of the output signal. With this technique, the output of the RF amplifier can be held at a relatively constant level over a wide range of input signal levels. However, changing the amplifier's gain over a wide range does not provide the desired linearity.
The RF amplifier 10 is designed to be used with a single RF frequency band, with the tuned circuit 15 being tuned to the desired band. Multi-band receivers generally require multiple tuned circuits, one for each frequency band to be received, plus switching devices to select the proper tuned circuit for the frequency band to be received. AGC is also preferably provided for all frequency bands, but including AGC control as shown in FIG. 1, in addition to the switching devices needed to select the proper tuned circuit, can lead to duplication of circuitry and an undesirably large number of components. The problem of reduced linearity also remains.